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Re: Uniquely identifying each & every html template

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Date Wed, 23 Jan 2013 19:09:52 -0500
From Dave Angel <d@davea.name>
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Subject Re: Uniquely identifying each & every html template
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On 01/23/2013 06:25 PM, Chris Angelico wrote:
> On Wed, Jan 23, 2013 at 11:38 PM, Dave Angel <d@davea.name> wrote:
>> You think it's an accident that md5 size is roughly equivalent to 39 decimal
>> digits?  Or that the ones that haven't been proven insecure are much larger
>> than that?  The sha512 hash is roughly equivalent to 154 decimal digits.
>
> Proving a hash function secure or not is orthogonal to its length. You
> could have a cryptographically secure hash function that produces a
> single byte; you'd get collisions pretty often, but that's understood.
> Conversely, you could have an insecure hash that produces a value
> several orders of magnitude longer than SHA512. Look at this:
>
> def big_long_hash(val):
>      return sum(bytes(str(val),"utf-8"))*12345678901234567890
>
> But longer hashes do reduce the chance of collisions, by the
> fundamental rules of mathematics.
>

I certainly can't disagree that it's easy to produce a very long hash 
that isn't at all secure.  But I would disagree that longer hashes 
*automatically* reduce chances of collision.

Anyway, about cryptographically ...

OK, I'd like to learn here.  I thought that "cryptographically secure" 
meant that it was infeasible to take a given message and make an 
innocuous change to it (such as adding a trailer of whatever size) and 
from that produce a predetermined hash value.

Obviously "infeasible" will change over time.  But if my definition is 
even close, then wouldn't it be a necessary (not sufficient) condition 
that the hash be at least some certain size.  It is that size I was 
trying to impress on the OP.

Wikipedia - http://en.wikipedia.org/wiki/Cryptographic_hash_function

seems to say that there are four requirements.
it is easy to compute the hash value for any given message
it is infeasible to generate a message that has a given hash
it is infeasible to modify a message without changing the hash
it is infeasible to find two different messages with the same hash


Seems to me a small hash wouldn't be able to meet the last 3 conditions.




-- 
DaveA

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Thread

Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-18 12:48 -0800
  Re: Uniquely identifying each & every html template John Gordon <gordon@panix.com> - 2013-01-18 20:59 +0000
    Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-18 14:12 -0800
  Re: Uniquely identifying each & every html template Dave Angel <d@davea.name> - 2013-01-18 17:09 -0500
    Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-19 00:39 -0800
      Re: Uniquely identifying each & every html template Dave Angel <d@davea.name> - 2013-01-19 04:00 -0500
        Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-20 23:08 -0800
          Re: Uniquely identifying each & every html template Chris Angelico <rosuav@gmail.com> - 2013-01-21 18:20 +1100
            Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 01:19 -0800
              Re: Uniquely identifying each & every html template Chris Angelico <rosuav@gmail.com> - 2013-01-21 20:31 +1100
                Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 04:06 -0800
                Re: Uniquely identifying each & every html template Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-01-21 12:39 +0000
                Re: Uniquely identifying each & every html template alex23 <wuwei23@gmail.com> - 2013-01-21 04:55 -0800
                Re: Uniquely identifying each & every html template rusi <rustompmody@gmail.com> - 2013-01-21 19:24 -0800
                Re: Uniquely identifying each & every html template Chris Angelico <rosuav@gmail.com> - 2013-01-22 15:39 +1100
                Re: Uniquely identifying each & every html template Tom P <werotizy@freent.dd> - 2013-01-22 00:01 +0100
                Re: Uniquely identifying each & every html template Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-01-21 23:43 +0000
                Re: Uniquely identifying each & every html template Chris Angelico <rosuav@gmail.com> - 2013-01-22 11:04 +1100
                Re: Uniquely identifying each & every html template alex23 <wuwei23@gmail.com> - 2013-01-22 17:36 -0800
                Re: Uniquely identifying each & every html template Joel Goldstick <joel.goldstick@gmail.com> - 2013-01-21 07:47 -0500
                Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 07:00 -0800
                Re: Uniquely identifying each & every html template Michael Torrie <torriem@gmail.com> - 2013-01-22 16:55 -0700
                Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-23 01:12 -0800
                Re: Uniquely identifying each & every html template alex23 <wuwei23@gmail.com> - 2013-01-23 01:37 -0800
                Re: Uniquely identifying each & every html template Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-01-23 09:49 +0000
                Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-23 02:29 -0800
                Re: Uniquely identifying each & every html template Joel Goldstick <joel.goldstick@gmail.com> - 2013-01-23 07:03 -0500
                Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-23 04:26 -0800
                Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-23 04:26 -0800
                Re: Uniquely identifying each & every html template alex23 <wuwei23@gmail.com> - 2013-01-23 06:25 -0800
                Re: Uniquely identifying each & every html template Dave Angel <d@davea.name> - 2013-01-23 07:38 -0500
                Re: Uniquely identifying each & every html template Chris Angelico <rosuav@gmail.com> - 2013-01-24 10:25 +1100
                Re: Uniquely identifying each & every html template Dave Angel <d@davea.name> - 2013-01-23 19:09 -0500
                Re: Uniquely identifying each & every html template Chris Angelico <rosuav@gmail.com> - 2013-01-24 11:39 +1100
                Re: Uniquely identifying each & every html template Dave Angel <d@davea.name> - 2013-01-23 19:53 -0500
                Re: Uniquely identifying each & every html template Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-01-23 16:01 -0500
                Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-23 01:12 -0800
                Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 07:00 -0800
                Re: Uniquely identifying each & every html template Dave Angel <d@davea.name> - 2013-01-21 17:26 -0500
                Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 04:06 -0800
                Re: Uniquely identifying each & every html template Tim Roberts <timr@probo.com> - 2013-01-21 19:57 -0800
              Re: Uniquely identifying each & every html template Tim Roberts <timr@probo.com> - 2013-01-21 20:04 -0800
                Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 22:49 -0800
                Re: Uniquely identifying each & every html template Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-01-22 16:08 -0500
                Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-23 01:15 -0800
                Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-23 01:15 -0800
            Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 01:19 -0800
              Re: Uniquely identifying each & every html template alex23 <wuwei23@gmail.com> - 2013-01-21 04:56 -0800
                Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 07:03 -0800
                Re: Uniquely identifying each & every html template alex23 <wuwei23@gmail.com> - 2013-01-21 15:35 -0800
              Re: Uniquely identifying each & every html template Piet van Oostrum <piet@vanoostrum.org> - 2013-01-21 21:48 +0100
                Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 22:38 -0800
            Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 07:07 -0800
              Re: Uniquely identifying each & every html template alex23 <wuwei23@gmail.com> - 2013-01-21 15:36 -0800
              Re: Uniquely identifying each & every html template rusi <rustompmody@gmail.com> - 2013-01-21 20:18 -0800
            Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 07:07 -0800
        Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-20 23:08 -0800
      Re: Uniquely identifying each & every html template Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-01-19 16:32 -0500
        Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-20 22:52 -0800
        Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-20 22:52 -0800
      Re: Uniquely identifying each & every html template John Gordon <gordon@panix.com> - 2013-01-22 16:35 +0000
    Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-19 00:39 -0800

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